In the Universal Mobile Telecommunications System (UMTS) network, the specifications of long term evolution (LTE) have been drafted for the purpose of further increasing high speed data rates, providing lower delays and so on (see non-patent literature 1). Also, successor systems of LTE (also referred to as, for example, “LTE-advanced” (hereinafter referred to as “LTE-A”), “FRA” (Future Radio Access) and so on) are under study for the purpose of achieving further broadbandization and increased speed beyond LTE.
The system configuration of LTE Rel. 10-12 includes at least one component carrier (CC), where the LTE system band constitutes one unit. Such bundling of a plurality of component carriers (cells) into a wide band is referred to as “carrier aggregation” (CA). In LTE Rel. 10-12 systems, CA to use maximum 5 CCs is utilized.
In LTE systems, hybrid automatic repeat request (HARQ: Hybrid Automatic Repeat reQuest) is supported in order to suppress degradation of communication quality due to signal reception failures in radio communication between a user terminal (UE) and a radio base station (eNB). For example, a user terminal feeds back delivery acknowledgment signals (referred to as “HARQ-ACKs,” and also referred to as “ACK/NACKs”) depending on the condition of reception of downlink (DL) signals transmitted from a radio base station. There is a rule that, when the user terminal transmits HARQ-ACKs in an uplink control channel (PUCCH), the user terminal uses predetermined PUCCH formats according to the number of CCs (or cells) and so on (see non-patent literature 2).